Prohibit Merkle Internal Node Preimages That Encode Minimal 64-Byte Transactions

The discussion revolves around the effectiveness of invalidating 64-byte transactions in preventing malleability issues within Bitcoin's protocol, specifically concerning SPV (Simplified Payment Verification) verifiers. The primary point made is that this method addresses malleability by distinguishing between a leaf and a node in transaction data structures. It was highlighted that this solution effectively mitigates malleability problems in one direction, which involves the confusion between leaves and nodes.

Further clarification provided posits that for SPV verifiers expecting proofs for non-64-byte transactions—those linked to significant scripts or where actual value transfer occurs—the change remains effective without additional adjustments needed from the verifiers. This assertion underscores that as long as it remains computationally impractical to manipulate the smaller transactions to mimic larger ones, the security of the SPV proofs holds.

Moreover, an example was given to illustrate how, even if 64-byte transactions were invalidated, a miner attempting to exploit this by faking an SPV proof would face significant computational challenges. Specifically, the attacker would need to grind through 256 bits to create a matching inner node, making the task virtually unfeasible. This suggests that the risk of such attacks does not increase with the invalidation of 64-byte transactions, thereby supporting the argument that this measure alone is sufficient to curb malleability issues without necessitating further action from SPV verifiers.

Overall, the conversation confirms that while BIP 54 claims that invalidating 64-byte transactions curbs malleability, it does so effectively without requiring changes from SPV verifiers, assuming these verifiers are aligned with the expectations set forth regarding transaction sizes and types they handle.